1. Field of the Invention
The present invention relates to an optical coherence tomography method and an optical coherence tomography apparatus, and more particularly to an optical coherence tomography method and an optical coherence tomography apparatus, using a coherent optical system for use in the medical field.
2. Description of the Related Art
Currently, there are a wide variety of ophthalmic devices using optical devices. Examples of such ophthalmic devices include anterior eye imaging apparatuses, retinal cameras and scanning laser ophthalmoscopes (SLOs). Among them, optical coherence tomography (OCT) apparatuses can obtain tomographic images of objects to be measured at high resolution, and therefore are becoming indispensable devices for outpatient medical treatments specialized for retinas.
An OCT apparatus is disclosed, for example, in Japanese Patent Application Laid-Open No. H11-325849. In an OCT apparatus disclosed in Japanese Patent Application Laid-Open No. H11-325849, low coherent light is used. Light from a light source is divided into measurement light and reference light through a split optical path, such as a beam splitter. The measurement light is applied onto an object to be measured, such as a human eye, through a measurement optical path, and return light from the object to be measured is led to a detection position through a detection optical path. The return light as used herein refers to reflected light or scattered light that includes information on an interface of the object to be measured with respect to the irradiation direction of light. The reference light is led to a detection position through a reference optical path. Input to a detection position is coherent light resulting from interference between the return light and the reference light. Then, the wavelength spectrum of the coherent light is collectively acquired by the use of a spectrometer or the like, and the wavelength spectrum is Fourier transformed, thereby obtaining a tomographic image of the object to be measured. In general, an OCT apparatus that collectively measures the wavelength spectrum is termed a spectral-domain OCT (SD-OCT) apparatus.
With an SD-OCT apparatus, the depth of focus and a transversal resolution can be adjusted by selecting a numerical aperture (NA) of a lens used for controlling a focusing position of the measurement light in an object to be measured. For example, the larger the numerical aperture, the smaller the depth of focus, but the higher the transversal resolution. On the other hand, if the numerical aperture is reduced, the depth of focus becomes larger, but the transversal resolution becomes lower. In other words, the relationship between the depth of focus and the transversal resolution is a trade-off.
As a method that overcomes the problems with this relationship, dynamic focus OCT is disclosed in “OPTICS LETTERS Vol. 28, 2003, pp. 182-184”. In this mode, time domain OCT (TD-OCT) that acquires a tomographic image while changing an optical path length is employed. Then, a tomographic image is acquired while changing the optical path length and moving the focus position of a lens in synchronization with each other. As a result, while the transversal resolution is maintained high, the measurement range of an object to be measured (the range in the irradiation direction of measurement light in an acquired tomographic image) can be increased.